The Roadside Survey of Alcohol and Drug Use by Drivers (PDF) indicates that drinking and driving has declined significantly – down almost 30% since last studied seven years ago. At the same time, however, drug use and driving has increased, with driving under the influence of marijuana up almost 50%.

A second report, the Drug and Alcohol Crash Risk Study (PDF), looked at whether marijuana use by drivers correlates to a higher risk of crashes. Results show that while marijuana users are more likely to be involved in crashes, the user population also overlaps with high risk drivers — namely, young males. When controlled for population, the likelihood drops to statistical insignificance, a conclusion that is prompting supporters of marijuana legalization to claim victory.

The fact that marijuana impairs is not debated, even among supporters. It affects judgment, reaction times and situational awareness. NHTSA administrator Mark Rosekind sums it up:

…we have much to learn about how illegal drugs and prescription medicines affect highway safety – and developing that knowledge is urgent, because more and more drivers have these drugs in their systems.

While some states allow citizens to use marijuana recreationally or medicinally, two things we know are:

Operators of commercial motor vehicles on U.S. roads and who cross state borders are regulated by Federal laws where marijuana use is illegal.

Impaired driving is illegal across the U.S., regardless of the legality of the drug (including alcohol) causing the impairment.

When discussing impaired driving, we need to understand that the body processes marijuana differently than alcohol and that testing also differs.

With marijuana, THC quickly enters the blood stream and is broken down into many (>60) metabolites. The THC and its metabolites are drawn into fatty tissues and organs with high blood flow (e.g., brain, heart, liver, etc.) Unlike alcohol which the body removes within hours, the metabolites of THC (THC-COOH, hereafter) are eliminated from the body slowly – taking up to four weeks. Heavy users may need far longer to clear the THC-COOH from their fatty tissues and organs.

Urine testing detects THC-COOH, but cannot identify when the THC was absorbed into the system. Typically, a urine test is representative of marijuana use in the past 30 days. Saliva and blood testing are effective in identifying un-metabolized THC and are used at some roadside sobriety checkpoints. However, no generally accepted measures exist to define levels of THC that correspond with impairment.

The issue raised by the Crash Risk Study is very complex. The insurance industry has long known that young males pose a higher risk as members of the driving public and charges premiums accordingly. Police statistics also show that young males are the largest users of marijuana.

The NHTSA study concluded that “marijuana use by drivers IS associated with a greater risk of crashes” (emphasis added). The issue with both studies is the inability to absolutely separate the likelihood of crashes due to being young and male from crashes due to marijuana use. Rather than showing any correlation between marijuana use and crashes, they simply show that more research is needed.

In the meantime, driving under the influence of marijuana presents a risk for any company whose employees drive for business. The presence of either THC or THC-COOH in a post-accident scenario muddies the question of liability. While NHTSA conducts further studies, to help mitigate that risk, companies need to have strong drug policies in place that include testing, counseling and training.

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